This invention relates to the field of interferometry, and particularly to scanning interferometers intended for use in spectrometry. More specifically, its primary focus is on improving Michelson interferometers intended for use in infrared Fourier transform spectroscopy.
The present invention is an improvement relating to the same general subject matter as two earlier filed applications of Walter M. Doyle, assigned to the assignee of this application. The first of the related Doyle applications is Ser. No. 790,497, filed Apr. 25, 1977, and titled "Refractively Scanned Interferometer". That application discloses and claims an interferometer having stationary reflectors at the end of each interferometer "arm" combined with a wedge-shaped prism movable across one arm to cause scanning.
The other of the related Doyle applications is Ser. No. 808,951, filed June 22, 1977, and also titled "Refractively Scanned Interferometer". That application discloses and claims an interferometer of the same type as Ser. No. 790,497, wherein the orientation and direction of motion of the wedge-shaped scanning prism are such that displacement of the refracted optical beam by the prism is minimized or eliminated.
The interferometers shown in the cited Doyle applications have provided a significant development in the field of Fourier transform spectroscopy. However, certain performance limitations have become apparent in those interferometers, which I attribute to certain optical aberrations not fully compensated for in the Doyle interferometers.
There are three general types of optical aberration encountered in apparatus of this type: (a) chromatic aberration, (b) spherical aberration, and (c) astigmatic aberration.
Chromatic aberration is partially compensated for in the Doyle apparatus by the use of corner reflectors. However, the compensation is only partial, and, further, there is an upper limit in such apparatus on the allowable distance between the scanning wedge and the corner reflector in the same arm. This limit in some cases is not acceptable.
Spherical aberration is minimized in the Doyle apparatus by matching the refractive thickness of the beamsplitter substrate to the refractive thickness of the scanning wedge in its centered position. In other words, the arrangement is such as to equalize the average optical path lengths in those two elements, which are located in different arms of the interferometer. While this arrangement, as predicted by Doyle, provides a reasonable solution of the spherical aberration problem, it tends to preclude solution of the astigmatism problem, for reasons which will be more fully discussed below.
Astigmatic aberration (astigmatism) was not compensated for in the apparatus of the Doyle applications, although it was later discovered that first order terms could be compensated for by a lateral displacement of one of the corner reflectors. Nevertheless, astigmatism has provided to be a limiting factor in the performance of the Doyle apparatus.
The general purpose of the present invention is to retain the significant advantages of the Doyle apparatus, while adding thereto the benefits of maximum compensation for, or balancing of, the three types of optical aberration.